"Something's wrong with the baby," said her boyfriend, Austin Hamilton. "He's not breathing."

She thought he was joking. Then dread overcame her. She told him to call 911 and bolted from her job at Phantom Fireworks on Nebraska Avenue. She raced down the block to the Motel 6, room 164.

On a bed, her baby lay clad in a diaper. His face was purple. His cheeks were sunken.

Hamilton hovered over him, a phone to his ear, the other hand pumping lightly on the boy's chest.

"What did you do to him?" she asked. He said nothing.

She grabbed the phone and listened to the 911 dispatcher, who talked her through CPR.

She put her hands against the boy's cold chest. She heard him gurgling. With each push, she felt a squishing beneath his skin.

Later that night, Sincere Williams died at St. Joseph's Hospital. A medical examiner's report concluded the 1-year-old succumbed to blunt-force trauma to the lower abdomen.

The next day, Hamilton, 24, was charged with his murder.

Detectives said he confessed to beating the boy with a leather belt more than 10 times and dropping him on his head. He did it, Tampa police said, because the boy urinated while Hamilton was changing his diaper.

• • •

Williams never thought Hamilton was capable of hurting her children. He had always been good with Sincere and the boy's 6-year-old brother, Douglas Moore. Her children had different fathers. Neither dad was in the picture.

But Hamilton seemed willing. He has a 4-year-old son of his own with another woman, Williams said.

The couple had known each other for years. But they didn't end up together until this year, when they began sharing the same house — the home of her oldest boy's grandmother.

Last week, the woman who owned the house where they were staying made them move out, Williams said. They stayed at the Motel 6 at 333 E Fowler Ave. They had been looking for an apartment, but Hamilton's criminal record made it hard.

He was charged with misdemeanor battery in 2012 but the case was later dropped, records show. The same year, a woman sought a domestic violence injunction against him, according to court records. It was denied after the woman failed to show up in court. A year earlier, child protection investigators conducted an investigation involving a child of Hamilton's, according to the Department of Children and Families. The nature of that investigation and its outcome were unclear.

Williams wasn't concerned. Hamilton had never been violent toward her or her sons. And he had previously watched the boys while she was gone, though there had always been someone else there, too.

She left the motel just before 11 a.m. She took a bus to Walmart to pick up a cashier's check for an apartment deposit. Then, she headed to work.

At 4 p.m., she sent Hamilton a text message. Everything was good, he replied. The older boy was watching TV. The baby was asleep. Hamilton said he had changed his diaper without waking him.

Williams later learned that the three had gone to a nearby Burger King at 6 p.m. She said police told her they pulled surveillance footage from the restaurant. On the video, Williams said, the older boy casts concerned looks at his baby brother, who sits motionless.

• • •

Williams wailed on the motel room floor as paramedics wheeled the boy into an ambulance. At the hospital, she got on her knees and begged God to let her switch places with her son. She watched through a window as Sincere's tiny chest sucked in air, the breaths getting slower and shallower.

She had felt this before, she said. Four years ago, she had a daughter who was stillborn. After that, doctors said she would never be able to have another child. Then, a year ago, came her second son. In her mind are images of him smiling at Mickey Mouse cartoons, and the funny faces his brother made as he ate cheese puffs.

Late Saturday, doctors told her there was nothing more they could do.

All the while, Hamilton leaned against a waiting room wall. It was cold in the waiting room. He asked for a blanket.

• • •

On Sunday, detectives questioned Hamilton.

After Williams left for work, he told them, he changed Sincere's diaper. In the process, the boy began to urinate all over the motel room.

Hamilton said he "lost control," and began to "spank" the boy with a leather belt, according to a police report. He hit him at least 10 times on his backside and "anywhere he could," the report stated. When he went to pick the boy up, Hamilton said, he dropped him on his head, the report stated.

Hamilton remained jailed Monday. A judge denied bail.

Williams hopes for the ultimate punishment.

"He'd better get death," she said. "He made my son suffer. Now he needs to suffer."

Times researcher John Martin contributed to this report. Contact Dan Sullivan at dsullivan@tampabay.com or (813) 226-3386. Follow @TimesDan.